Demacs is an extensible way of defining various things. Demacs is written
from scratch with the idea of syntax of cl-def in mind. Despite there are
some other definer utilities in the wide (e.g. definer, cl-def), almost
all of them lack of at least one of below functionalities.

Affordable dependency. (cl-def[1] depends on some libraries, and those
libraries depend on some other libraries and so on... You're required to
install a dozen of libraries to benefit from a single definer utility.
Furthermore, a major portion of related libraries are not available via
ASDF-INSTALL.)

To summarize, demacs is implemented with KISS principle in mind.
And as a bonus, demacs has no dependencies.